Controller for the back pressure against the main ram of a press



Aug. 29, 1961 F. J. KENLINE 2,998,238

CONTROLLER FOR THE BACK PRESSURE AGAINST THE MAIN RAM OF A PRESS Filed Sept. 11, 1959 BRIDGE.

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nited States Patent Ofice Patented Aug. 29, 1961 2,998,238 CONTROLLER FOR THE BACK PRESS AGAINST THE MAIN RALM OF A PRESS Frederick J. Kenline, Buffalo, N.Y., assignor to Lake Erie Machinery Corporation, Buffalo, N.Y., a corporation of New York Filed Sept. 1'1, 1959, Ser. No. 839,495 2 Claims. (Cl. 2671) This invention relates to a system for controlling the back pressure exerted by a liquid cushioning body against the main ram of a press and is more particularly shown in conjunction with a hydraulic press for pressing metal shapes and including a resilient pad or cushion into which the metal blank is forced by a punch under controlled pressure, the blank wrapping around the punch to take a corresponding shape. Such apparatus is particularly useful with unusual or complicated shapes which can be formed in one operation with such apparatus but which would require a number of separate operations with a corresponding large number of dies if done with male and female dies. With such unusual or complicated shapes, it is important that the pressure impressed on the workpiece be programmed or varied to correspond to changes in the shape of the punch as it progresses into the cushion, the formation of sharp corners, for example, requiring higher pressure. Such programming is effected by a template shaped to correspond to the changes in shape of the punch and the present invention relates to a hydraulic-electric circuit for controlling the back pressure exerted against the main ram by a body of cushioning liquid.

One of the principal objects of the present invention is to provide such a control system which will handle large volumes of liquid or oil without substantial overrun so that proper pressure is supplied at all times to correspond to the changing shape of the piece being formed.

Another object is to provide such a control system which is sensitive and quickly responds to changes in shape of the template.

Another object is to provide such a circuit in which the operating pressures can be predetermined and preset so that the particular workpiece will be formed under optimum changing pressures.

Another object is to provide such a control system which permits faster operation of the press.

Another object is to provide a controller which operates in response to a comparison of voltages, the voltages compared being one which is changed by a template moving with the main ram and another changed in re sponse to pressure changes in the cushioning body of liquid.

Other objects and advantages of the invention will be apparent from the following description and accompanying drawing which is a simplified vertical central section through a press having a main ram working against a cushion ram in conjunction with a resilient pad into which the workpiece is forced by a punch and equipped with a system controlling the back pressure against the main ram in accordance with the present invention.

In the drawing, the numeral 1 represents a main ram movable downwardly toward a punch 2 suitably mounted on punch bolster 3 which in turn is mounted on cross head 4 carried by the stationary frame or bed 5 of the press. The main ram 1 carries a resilient pad holder 6 having a downwardly opening large pocket or recess 8 containing a resilient pad 9 Which is in line with the punch 2.

The workpiece 10 is mounted on a draw ring 11 having a central opening 12 through which the punch 2 works and this draw ring is mounted on pins or columns 13 which are slidingly mounted for vertical movement in the punch bolster 3 and cross head 4 and at their lower ends butt against a cushion ram 14. This ram works in the open upper end of a cushion cylinder 15 containing a body 16 of hydraulic fluid. A fluid line or pipe 18 extends from the bottom of this cylinder to a servo valve 19 this servo valve controlling the relief of hydraulic fluid to a pressurized tank 20. This tank 20 can be pressurized through an air pressure line 21 under control of a valve 22. A branch line 23 from the line 18 connects with the bottom of the body of fluid 24 in the pressurized tank 20, this branch line containing a check valve 25 opening toward the cushion ram cylinder 15.

The numeral 30 represents an arm on the cushion ram 14 to which is attached a suitable means for driving a position indicating transducer. As shown here the arm is carrying a gear rack 31 driving the gear 32 of a syncro master unit 33. The output of this syncro master unit 33 is amplified by an amplifier 34 and controls a syncro slave unit 35. Through a gear 3 6 this syncro slave unit drives a rack 38 carrying a template 39.

This template 39 has a suitable shape 40 to program the cushion pressure for the punch 2. If the punch had sharp corners, which would require higher back pressure to form, the working edge or profile 40 would take a corresponding sharply deviating form.

This working edge '40 of the template 39 actuates a spring offset transducer here shown for purposes of explanation as a potentiometer '41 having an actuating arm 42 with a roller 43 yieldingly held in contact with this working edge or profile 40 of the template 39 and actuating a movable contact 44 traversing a resistor 45. The movable contact 44 and one end of the resistor 45 are connected by lines 46 and 48 across one of the four sides of a suitable electric bridge 49.

A pressure cell or transducer 50, through lines 51 and 52, is across a second side of this bridge 49. This transducer 50 is in the form of a pressure cell the change in pressure against which provides a corresponding change in value of an electrical resistor across the lines 51 and 52. This transducer is responsive to pressure changes in the cushion cylinder 15.

Power is supplied across the bridge 49 from power lines 53 and 54 under control of a switch 55.

The output from the bridge 49 is amplified by an amplifier 56 and controls a servo valve drive 58. Through lines 59 and 60 this servo valve drive actuates the servo valve 19. The amplifier 56 is preferably provided with a sensitivity control 61.

In operation, with the workpiece or blank 10 in position on the draw ring 11, the main ram 1 is moved down to engage its rubber cushion or pad 9 with this workpiece and to move the workpiece and draw ring downwardly. This movement is transmitted by the pins or columns 13 to the cushion ram 14 to force this cushion ram into the cushion cylinder 15.

At this time the servo valve '19 is closed and hence the oil in the cushion cylinder v15 cannot escape. Accordingly the downward pressure of the main ram 1 builds up the fluid pressure in the cushion cylinder. When the pressure is high enough the transducer or pressure cell 50 causes the polarity of the bridge unbalance to reverse. This unbalancing of the bridge alters its output through the amplifier 56 and servo drive 58 to open the servo valve 19 and thereby limit the build-up of cushion'ram pressure. When the cushion cylinder pressure is correct the output of the pressure cell is just right to balance the bridge thus reducing its output to zero and maintain a status quo in the position of the servo valve.

The downward movement of the cushion ram 14, through the rack 31, master and slave syncro units 33, 3-5 and rack 38, moves the template 39 downwardly. The roller 42 is moved by the profiled working edge 40 of this template to actuate the potentiometer 41 to again unbalance the bridge 49. This unbalance of the bridge calls for further opening or closing of the servo valve 19 through the amplifier 56 and servo valve drive '58.

' Accordingly pressure again builds up or reduces in the cushion cylinder 15 and again actuates the transducer or pressure cell 50 to rebalance the bridge when the cushion cylinder pressure reaches a value corresponding to the new position of potentiometer 41.

This unbal-ancing of the bridge 49 by the template 39 and rebalancing or correction of it by the transducer or pressure cell 50 continues until the workpiece is fully formed, the resilient pad 9 wrapping it about the plunger 2 and in doing so drawing it 'fi'OIII between the pad 9 and draw ring 11 the back pressure of which latter is regulated by the template 39 to correspond to the requirements of the shape of the punch.

The servo valve 19 is an intermediate acting valve, that is, opened a certain amount but, as described above, is opened only after the bridge is unbalanced by the potentiometer 41 stops opening (or closing) when the bridge is rebalanced by the transducer or pressure cell 50. This unbalancing and correction is, of course, rapid, much like a hysteresis lag, and provides a sensitive control for large volumes of liquid at high pressures without overrun and wherein the proper pressure can be applied at the right time with certainty.

When the pressing is completed and the main ram 1 retracted, the pressure on the body 24 of liquid in the pressurized tank 20 flows through the branch line 23 past its check valve 25 back to the cushion cylinder for a repetition of the sequence as above described.

While a cushion ram and cylinder 14, 15 are shown, it is apparent that the control system could be used to control the escape of a cushioning body of liquid disposed directly behind the resilient pad 9 as disclosed in Hottmann Patent 2,783,728 dated March 5, 1957, for Apparatus for Pressing Sheet Metal Shapes. In this patent the template mechanically relieves the fluid pressure behind the rubber pad, such mechanical control system being,

however, liable to overrun especially where large volumes of liquid are to be relieved, and also being subject to the disadvantage of not knowing or being able to predetermine the pressure at each stage of operation.

What is claimed is:

1. In a press having a main ram working against a liquid cushioning body having a fluid outlet, the combination therewith of a controller 'for the back pressure exerted by said cushioning body against said main ram, comprising a suitable electrical bridge, a template moved in response to the movement of said main ram and having a working profile corresponding to the pressure program required for the shape to be produced by the press, a potentiometer in one side of said bridge, means actuating said potentiometer in response to changes in said working profile as said main ram closes, a transducer responsive to the pressure in said cushioning body and having its output in a second side of said bridge, a power source to said bridge, and means responsive to the output across said bridge controlling the escape of fluid from said outlet under pressure exerted thereon by said ram.

2. In a press having a main ram working against a liquid cushioning body having a fluid outlet, the combination therewith of a controller for the back pressure exerted by said cushioning body against said main ram, comprising a suitable electrical bridge, a template moved in response to the movement of said m-ain ram and having a working profile corresponding to the pressure program required for the shape to be produced by the press, a potentiometer in one side of said bridge, means actuating said potentiometer in response to changes in said working profile as said main ram closes, a transducer responsive to the pressure in said cushioning body and having its output in a second side of said bridge, a power source to said bridge, valve means in said fluid outlet, and means responsive to the output across the output of said bridge and actuating said valve means in response to changing pressure exerted by said ram on said cushioning body.

References Cited in thefile of this patent UNITED STATES PATENTS 

